Impregnated blade coating

ABSTRACT

A blade for treating fibrous nonwoven webs having a base body and a coating covering at least a part of the surface of the base body. The coating forms at least that part of the blade surface which is provided to come into contact with the fibrous nonwoven web. At the contact surface, the coating has an open-pored material, the pores of which are filled at least partly with a polymer.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of PCT application No. PCT/EP2011/070659, entitled “IMPREGNATED BLADE COATING”, filed Nov. 22, 2011, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to blades for machines for the production of paper and, in particular to the configuration of blades for use in creping or coating of a paper web.

2. Description of the Related Art

In order to produce the surface properties required for a particular application purpose of a given paper type, the fibrous nonwoven web, or the so-called raw paper web, produced in the paper manufacturing process must, as a rule, be subjected to a separate treatment. In this regard, high quality paper types are coated and hygienic papers are creped.

The creping process produces paper types having surface structures. Normally a blade called a creping doctor is applied with its working edge against the surface of a drying cylinder or a Yankee cylinder (steam-heated cylinder, usually having a diameter of several meters), thereby stripping the web off the cylinder. To produce the surface structure, the creping doctor has an abutting surface located near its working edge where the web is backed up and thereby structured at impact.

For smoothing the web surface a mostly paste-like coating layer consisting of pigments, binding agents and additives is applied. Coating of the web can occur in a separate process. It is, however, usually integrated into the paper manufacturing process through an in-line coater. Smooth paper surfaces are achieved in a coating process whereby the coating is applied onto the paper and whereby the excess coating medium is subsequently removed by a blade. Due to the pressure exerted by the blade or respectively the doctor blade, recesses in the paper surface are filled with the coating medium, thus achieving a uniform surface of the coated paper.

Since in the two processes strong forces act upon the blades, high demands are placed on the mechanical stability and wear resistance of the blades. Creping blades and doctor blades are therefore normally equipped with a coating in the affected regions, the coating having a higher wear resistance or respectively a lower wear rate than the base material of the blade. These types of coating are usually produced from an abrasion resistant material by using metal oxides or hard metals in which metal carbide is embedded in a cobalt-, nickel- or iron matrix. In order to apply coatings which are physically as well as chemically as homogeneous as possible, thermal spray techniques are preferably used, whereby the coating medium is often applied in several passes. Each of the passes applies a thin layer of coating medium onto the base material of the blade or respectively onto the last coating layer applied thereto. Application of the coating in several thin layers ensures that the components of the coating medium cannot separate during build-up of the coating. In maintaining the coating parameters, a macroscopic homogeneous coating can be produced based on the chemical as well as the physical identity of the individual layers.

However, the wear and tear characteristics of these coating mediums do not permit extended standstills under the conditions prevailing during operation, so that creping doctors and doctor blades still need to be replaced in very short intervals in order to maintain the paper quality. Particles and contaminants on and inside the paper web moreover lead to localized load peaks on the blades which regularly cause damage on the blade coating, thereby reducing the quality of the produced paper.

What is needed in the art is a blade coating for creping doctors and doctor blades which possesses an improved wear resistance.

SUMMARY OF THE INVENTION

The present invention provides a blade for treating fibrous nonwoven webs, the blade having a base body and a coating covering at least part of the surface of the base body, wherein the coating forms at least that part of the blade surface which is provided to come into contact with the fibrous web, and whereby the coating on the contact surface consists of an open pored material, the pores of which are filled at least partially with a polymer.

A blade according to this specification has good gliding properties and a dirt-repellent effect on the contact surface intended for contact with an object traveling along it. These properties diminish the forces acting upon the coating, thereby extending the lifetime of the coating and reducing the risk of damage during service of the blade. The dirt repelling effect of the contact surface prevents agglomeration of particles on or near the coating surface which is in contact with the paper web and thereby improves the produced paper quality.

It is pointed out that the terms used in this description and in the claims in referring to characteristics such “comprise”, “have”, “include”, “contain” and “with” as well as grammatical variants thereof are generally to be understood as non-limiting in listing of devices, locations, sizes, etc. and do not in any way exclude the presence of other or additional properties or groupings of other or additional properties.

According to one embodiment of the present invention, the polymer includes an epoxy resin, because this effectively moistens the open pored coating material in its non-cross-linked or partially cross-linked condition, thereby being able to penetrate deeply into the pores of the coating. In additional embodiments the polymer advantageously includes a silicone-polyester resin, since this combines very good anti-sticking properties and thereby sliding properties with an excellent dirt repellent effect. To improve the sliding or dirt-repelling properties of the contact surface of the coating, fillers may be embedded into the polymer, whereby the fillers of preferred design variations thereof contain poly fluorinated ethylene (PFE) and in particular polytetrafluoroethylene (PTFE). The fillers may, for example, be present in the form of particles and particularly in the form of particles having median diameters from within the range of approximately 0.1 to 5 micrometers (μm). Instead of consisting of an epoxide resin or silicone polyester resin with fillers containing PFE or PTFE, the polymer may also be formed directly of poly fluorinated ethylene and in particular of polytetrafluoroethylene or a polymer which includes such a substance. To bring polytetrafluoroethylene into the pores of the coating, polytetrafluoroethylene particles of sizes from within the range of approximately 50 to 100 nanometers (nm) are, for example, prepared into a slurry and the thus obtained dispersion is applied into the pores, for example with the assistance of immersion, spraying or application with a brush or another coating device. A polymer may be used for this which is present in the form of particles in various sizes, whereby at least 65 percent of the particles are of one or more sizes from the range of 50 to 100 nm.

The open-pored base material of the coating in all embodiments consists of an oxide ceramic or of a mixture of two or more oxide ceramics. Additional embodiments feature an open pored coating base material which includes one or more oxide ceramics. In some embodiments, the oxide ceramics are selected from aluminum oxide, zirconium dioxide, magnesium oxide, chromium (III) oxide, yttrium oxide and titanate, which combine good mechanical stability with high abrasion resistance and which can be applied with modern high speed spray methods, for example High Velocity Oxygen Fuel (HVOF) onto the base body of the blade. In additional embodiments the open pored material of the coating includes a hard metal, which can also combine good mechanical stability with high abrasion resistance and which can be effectively and economically applied with modern high speed spray methods. The porosity of the coating material is, for example, between approximately 2 and 10 percent, whereby these values reflect the area proportion of the pores interspersed cross-sectionally in the material. The median pore diameter of the pores of the coating base material may have a value from the range of 5 to 15 μm. To improve adhesion of the coating on the base body of the blade, an adhesive layer may be arranged between them.

A blade as previously described may be utilized as a creping doctor with a working edge and an abutting surface, or as a doctor blade.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic cross section through a creping doctor in the region around the working edge; and

FIG. 2 is a schematic cross section through a doctor blade in the region of its facet.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there is shown a cross section through the front section of creping doctor 10 which is provided with coating 12. Creping doctor 10 has a base body 11 which is formed, for example from steel, onto which coating 12 is applied. Coating 12 occupies at least that part of creping doctor 10 which comes into contact with the dryer- or Yankee cylinder and the fibrous nonwoven material of the web. As shown in FIG. 1, an intermediate layer 13 may be provided between coating 12 and base body 11 for better adhesion.

Working edge 14 with which creping doctor 10 fits closely against the dryer- or Yankee cylinder abuts to abutting surface 15 onto which the paper web impacts. Abutting surface 15 and working edge 14 are positioned at the location of the coating.

As can be seen in schematic detail A of coating 12 in the region of working edge 14, coating 12 includes two components. One coating matrix or coating support layer 16 formed by a coating base material in which smallest hollow spaces 17 are formed, and one polymer arranged in hollow spaces 17 of coating matrix 16. The density of the hollow spaces is approximately constant in the illustrated embodiment of the present invention. The hollow spaces develop during application of the coating material onto base body 11 or onto bonding agent layer 13 and are called pores. The density of pores 17 does not have to be approximately constant as shown, but can change from the surface of the coating in the direction toward the base body, for example in order to facilitate a speedy breaking in of creping doctor 10. The coating matrix is open pored, whereby the term “open pored” is to be understood that hollow spaces positioned lower in the matrix are connected with hollow spaces located on the surface of the coating. The porosity of coating layer 12, that is the ratio of the pore volume relative to the total volume of coating 12 is strongly exaggerated in FIG. 1 for illustrative purposes. As a rule, coating 12 has a porosity in the range of approximately 2 to 10 percent, whereby these values, as already mentioned, reflect the area proportion of the pores penetrated cross-sectionally in the material.

To produce coating 12, the coating material is first applied onto base body 11 or, if a bonding agent is used, onto bonding agent layer 13 which was applied earlier using a thermal spray process onto base body 11. The choice of materials suitable to produce the bonding agent layer is determined by the material used for the base body, as well as the base coating material used in a particular instant. If base body 11 consists, for example, of steel at the edge which is to be coated, then the material for bonding agent layer 13 can be selected from the following coating materials: aluminum, nickel, chromium and alloys thereof, for example AlNi or NiCr.

To apply the base coating material, a thermal spray process may be used. The spray coating occurs hereby in several passes, for example in 10 to 100 passes. Each pass produces a thin layer of the coating material, whereby the first layer is sprayed directly onto the surface of base body 11 or onto the previously applied bonding agent layer, and additional layers are sprayed onto the respective previously applied layer 13. The physical homogeneity or vice versa the porosity of the individual layers can be adjusted through the parameters of the used process. For example, with the spray process known by the acronym HVOF (high velocity oxygen fuel) the porosity can be adjusted through the ratio of fuel to oxygen and through the feed rate of the powder material used for layer formation. By changing the parameters from layer to layer the porosity can be changed across the layer depth in regions mentioned above.

Commercial hard metal powders containing approximately 8-10% cobalt and tungsten mono-carbide as hard material are suitable as base coating material. To produce ceramic coatings, powders of oxide ceramic materials, for example, aluminum oxide, zirconium oxide, magnesium oxide, chromium (III) oxide, yttrium oxide and titanate may be used.

After application of the base coating material its open pores are filled with a polymer. Filling of pores in this application is understood to be the introduction of material into the pores, whereby the introduced material does not necessarily need to fill the respective pore volume, but can do so. When partially filling the pores, the introduced material may settle on the pore walls, however it may also be disposed in the pore volume, totally or partially removed from the pore walls.

Thermosetting plastics and thermoplastics are suitable as polymers which can be produced on the basis of single component or two-component systems. Exemplary thermoplastics which are suitable are thermosetting plastics whose decomposition temperature is so far above the service temperature of the blade coating that the thermosetting plastic reacts in a flexible manner. Analogously, thermoplastics whose glass transition temperature is so far above the service temperature of the blade coating, that during use of the blade which is impregnated with the polymer, no disturbing softening of the polymers can occur. As a rough reference value for the minimum difference between service temperature and decomposition or glass transition temperature, 20° C. can be cited. Service temperature is to be understood to be the operating temperature of the blade coating during intended use of the blade.

Epoxy resins and epoxy resins with filler particles, for example consisting of poly-fluorinated ethylene (PFE) and in particular polytetrafluoroethylene (PTFE) embedded therein are especially suitable as polymers. Since epoxy resin in non-cross-linked or partially cross-linked condition shows good wetting of the coating base material it can—for example supported by capillary effect—penetrate deeply into and fill the open pores of said coating base material. The viscosity of the epoxy resin can be reduced by the addition of solvents, for example alcohols or ketones in order to adapt the penetration depth of the polymer to the thickness of the coating. The impregnation process, in other words the introduction of the material into the pores of the blade coating, can be conducted with the assistance of immersion, spraying or application with coating devices, for example brushes or spatulas.

In an exemplary embodiment of creping doctor 10, an approximately 50 μm thick NiAl₅ bonding agent layer 13 is applied onto a surface region of base body 11 consisting of steel, by use of a thermal spray process. Subsequently a Cr₂O₃-ceramic is applied onto the free surface of bonding agent layer 13 using a plasma spray process, for example the above referenced HVOF process. The Cr₂O₃-powder used for the application has a granular size distribution whereby the granule size which is not exceeded by 90% of the hard material granules is at least twice as large, for example at least three times as large, as the granule size which is not exceeded by 10% of the hard material granules, whereby in particular a granular size distribution of 15/45 wherein 90% of the powder granules are no larger than 45 μm and 10% of the powder granules are no smaller than 15 μm is preferred.

The thickness of the applied Cr₂O₃-layer according to the exemplary embodiments is approximately 300 μm, the porosity of the layer approximately 2 to 3%, whereby the median pore diameters are in the range of approximately 5 to 15 μm. Pore diameter is hereby to be understood to be the diameter of a circle whose surface content is consistent with the pore cross section at the respective position. The hardness of a Cr₂O₃-coating layer produced in this manner can be specified with approximately 1150 HV 0.3 (Vickers hardness expressed in HV). After application of the Cr₂O₃-coating a filler material is introduced into its pores. The filler material consists of silicone-polyester resin mixed with iso-butanol which contains PTFE particles of median sizes of 0.1 to 5 μm. The proportion of the silicone polyester resin in the filler material is between 40 and 70 weight-%, that of the iso-butanol between 10 and 60 weight-% and that of the PTFE particles between 2 and 20 weight-%.

A further embodiment of the present invention differentiates itself from the one described above in the selection of the filler material. The filler material in this embodiment consists of a mixture of epoxy resin and iso-butanol which contains PTFE particles having a median diameter of 0.1 to 5 μm. The proportion of the epoxy resin in the filler material is again between 40 and 70 weight-%, that of the iso-butanol between 10 and 60 weight-% and that of the PTFE particles between 2 and 20 weight-%.

The depth to which the filler material penetrates into coating matrix 16 is influenced by the viscosity of the filler material and the temperature of the coating matrix during the filling process. In the case of high solvent contents the viscosity of the filler material is low, whereby the pores can be filled to great depth. By heating the coating matrix during filling, the maximum penetration depth of the filler material can be further increased provided that the temperature is sufficiently below the cross-linking temperature. Penetration temperatures in the range of approximately 70 to approximately 90° C., for example around 80° C. are suitable for the specified filler material systems, whereby with the above specified median pore diameters of approximately 5 to 15 μm, penetration depths of approximately 800 to approximately 1000 μm can be achieved.

After penetration of the filler material into the pores of coating matrix 12 it is, for example, thermally cross-linked, whereby the impregnation of coating matrix 16 is concluded. Lastly, the free surface of the impregnated coating 12 is fine ground in order to provide a smooth abutting surface and a smooth working edge. In such a grinding process, approximately 50 μm coating material are normally removed. The described sequence of impregnation and fine grinding is not mandatory and can, if necessary, also be carried out in reverse.

Due to the polymer which was introduced into the coating matrix, fibrous nonwoven web and dryer- or Yankee cylinder can slide along the respective contact surfaces of the creping doctor with low friction. The abrasion of the coating is reduced due to the increased gliding quality of the contact surfaces. The contact surfaces moreover achieve a dirt-repellent effect, thereby reducing damage caused by particles carried along by the fibrous nonwoven or the dryer- or Yankee cylinder and reducing contamination, thereby improving the quality of the crepe paper.

The schematic illustration in FIG. 2 shows a cross section through the front region of doctor blade 20 with coating 22. The doctor blade has base body 21 which may, for example, consist of steel, as well as coating 22 and bonding agent layer 23 arranged in some of the embodiments between them. Coating 22 occupies at least that region of doctor blade 20 which comes into contact with the fibrous nonwoven web or respectively with a coating material applied thereon. Doctor blade 20 moreover has a chamfer which is generally referred to as the facet of the blade. As a rule, coating 22 covers base body 21 as shown in the drawing, also in the area of the facet.

Apart from the geometry, the coating structure of the doctor blade is consistent with that of the creping doctor. In other words, the coating consists of a porous coating matrix as described above whose pores are filled at least partially with a polymer, also as specified above. In using this type of doctor blade the contact surface of the coating toward the web has an improved gliding quality and dirt-repelling effect, thereby reducing the probability of damage to the coating which could manifest itself on the coated paper in the form of line-type irregularities or micro-lining.

Due to the better gliding properties and dirt repelling effect of the coating surface, less dirt and coating material settle during use of the doctor blade on its side opposite the paper side, thereby clearly reducing the threat of the wet and dry boiling over, as well as beard-like build-up on the blade tip.

Blades according to the described invention have an improved abrasion resistance due to the better gliding properties on the coating surface, than blades with coatings that are not accordingly impregnated. The impregnation also reduces the probability of damage to the coating during operation and due to its dirt-repellent effect improves the quality of a paper web being processed with the blade.

While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims. 

What is claimed is:
 1. A blade for treating a fibrous nonwoven web, the blade comprising: a base body having a surface; a coating covering at least part of said surface of said base body, said coating forming at least a part of a surface of the blade which contacts the fibrous nonwoven web, said coating having on a contact surface an open pored material including a plurality of pores, said pores being filled at least partially with a polymer.
 2. The blade according to claim 1, wherein said polymer is an epoxy resin.
 3. The blade according to claim 1, wherein said polymer is a silicone polyester resin.
 4. The blade according to claim 3, further comprising a plurality of fillers embedded in said polymer.
 5. The blade according to claim 4, wherein said fillers contain fluorinated ethylene.
 6. The blade according to claim 5, where said fluorinated ethylene is polytetrafluoro ethylene.
 7. The blade according to claim 4, wherein said fillers are a plurality of particles.
 8. The blade according to claim 7, wherein said plurality of particles have a median diameter in the range of between approximately 0.1 and 5 micrometers (μm).
 9. The blade according to claim 1, wherein said polymer includes fluorinated ethylene.
 10. The blade according to claim 9, wherein said polymer includes polytetrafluoro ethylene.
 11. The blade according to claim 10, wherein said polymer is in the form of a plurality of particles having a size distribution, at least 65 percent (%) of said plurality of particles of said polymer being of at least one size in a range between 50 and 100 nanometers (nm).
 12. The blade according to claim 1, wherein said open-pored material of said coating includes one oxide or a plurality of oxide ceramics.
 13. The blade according to claim 12, wherein said open pored material of said coating is one of aluminum oxide, zirconium dioxide, magnesium oxide, chromium (III) oxide, yttrium oxide and titanate.
 14. The blade according to claim 1, wherein said open-pored material of said coating includes a hard metal.
 15. The blade according to claim 1, wherein said open-pored material has a porosity in a range of between approximately 2% and 10%.
 16. The blade according to claim 1, wherein a median pore diameter of said plurality of pores has a value in a range of from approximately 5 to 15 micrometers (μm).
 17. The blade according to claim 1, further comprising a bonding agent layer arranged between said coating and said base body.
 18. The blade according to claim 1, wherein the blade is configured as a creping doctor including a working edge and an abutting surface.
 19. The blade according to claim 1, wherein the blade is a doctor blade. 